1. Field of the Invention
This invention relates to a radio transmitter including a frequency modulator or an FSK (frequency shift keying) modulator using a PLL (phase-locked loop).
2. Description of the Related Art
Some frequency modulators or FSK (frequency shift keying) modulators include a PLL (phase-locked loop) using a signal generated by a VCXO (voltage controlled crystal oscillator) as a reference-frequency signal. A modulating signal is inputted to a VCO (voltage controlled oscillator) forming a part of the PLL. The VCO serves to modulate a carrier in accordance with the modulating signal to generate a modulation-resultant signal.
In some cases, for implementing modulation in a wide frequency range from the direct current to several kHz, the VCXO is used to modulate the reference-frequency signal in accordance with the modulating signal. In these cases, there are two modulation paths, that is, a VCO-based modulation path and a VCXO-based modulation path.
The VCO-based modulation path and the VCXO-based modulation path have different frequency responses. It is known to compensate for such a frequency-response difference between the two modulation paths. Specifically, Japanese patent application publication number 2000-341046 discloses a frequency modulation (FM) circuit including a VCO-based modulation path and a VCXO-based modulation path in which one of the two modulation paths is designed to adjust the balance in frequency-dependent modulation index between the two modulation paths.
It is desirable that the frequency deviation (shift) caused by the VCXO is sensitive to a control voltage applied thereto for making the modulation by the VCXO proper. Generally, it tends to be difficult to attain conformity with the standards for system frequency variation while considering a change in the frequency of a signal outputted from the VCXO which is caused by a change in the ambient temperature.